See this image:
What is he doing there? Why is he aiming at the Millennium Falcon?
To avoid active scanning - from planet-bound scanners - that could be detected from Imperial space probes, the rebel is using a hand scanner to read the transponder code / weapons status / access code - you name it - of the Falcon.
What you don't see is that he calls then the base for confirmation.
This individual is identified on the Starwars.com databank website as Corporal Osleo Prennert. Apparently he was acting as a sentry for the base. The device in his hand appears to be some kind of scanner rather than a gun.
Corporal Osleo Prennert was the sentry on duty when the Millennium Falcon landed on Yavin 4.
And from the film's junior novelisation
...a Rebel sentry visually monitored the freighter’s atmospheric descent from an observation tower, which was little more than a barrel-topped pole that extended high above the jungle floor